1. Field of the Invention
This invention relates to a method for forming a deposited film, which comprises forming a photoconductive film, a semiconductor film or an insulating film on a certain substrate by utilizing light as the excitation energy, and in detail to a method for forming a deposited film particularly of amorphous silicon (a-Si) on a certain substrate by creating an excited and decomposed state of a starting gas by imparting or utilizing light and, if desired, heat.
2. Description of the Prior Art
In the prior art, as the method for forming a deposited film of a-Si, there have been known the glow discharge deposition method and the heat energy deposition method employing SiH.sub.4 or Si.sub.2 H.sub.6 as the starting material. That is, these methods comprise forming a deposited film of a-Si on a substrate by decomposing SiH.sub.4 or Si.sub.2 H.sub.6 as the starting material by electric energy or heat energy (excitation energy), and the deposited film has been utilized for various purposes such as a photoconductive film, a semiconductor film, an insulating film or the like.
However, in the glow discharge decomposition method wherein the formation of a deposited film is conducted under high output discharging, it is difficult to control the condition with stability and reproducibility, because uniform distribution of discharging cannot be always obtained, and further because the high output discharging has a great effect on the film during film formation. As the result, it is difficult to ensure uniformity in electrical and optical characteristics as well as stability in quality of the formed film, and disturbance of the film surface during deposition or defects within the deposited film is liable to occur. Particularly, it has been very difficult to form a deposited film with large area or great thickness having uniform electrical and optical characteristics according to this method.
On the other hand, in the heat energy deposition method, since a high temperature of 400.degree. C. or higher is generally required, available substrate materials are limited. Besides, since the probability of elimination of the useful bonded-hydrogen atoms in a-Si is increased, desired characteristics can be obtained with difficulty.
Accordingly, as one method of overcoming these problems, the light energy deposition method (photo-CVD) employing SiH.sub.4 or Si.sub.2 H.sub.6 as the starting material is recently attracting attention.
The light energy deposition method employs light as the excitation energy in place of glow discharge or heat in the above methods, and preparation of a deposited film of a-Si at a low energy level has been rendered practicable. Also, light energy can be easily irradiated uniformly on the starting gas, and thereby film formation of high quality can be performed while maintaining uniformity with a lower energy consumption as compared with the deposition methods as described above. Also, the preparation conditions can be easily controlled, and the stable reproducibility can be obtained, and since the substrate is not required to be heated to a high temperature, the scope of the choice of substrates can be broadened.
However, in the light energy deposition method employing SiH.sub.4 or Si.sub.2 H.sub.6 as the starting material, decomposition with a dramatically good efficiency can be expected only to a limited extent. Accordingly, it has been pointed out that film-forming speed cannot be improved, thus posing a problem with respect to bulk productivity.